If there is one thing hardened criminals are ever likely to admit, it is that life in prison can be tough.
At best, it means being kept in a confined space surrounded by unpleasant company while having little chance of escape.
But when torture, starvation, disease and routine murder are added to the mix, the ordeal can be truly terrifying.
Below are five of the toughest jails around the world.
5. Rikers Island Prison, New York, USA
Rikers Island, a formidable prison fortress located in New York City’s East River, is a notorious hotbed of violence known for particularly brutal guards.
More recently, wardens have been accused of encouraging a fight club among the 14,000 inmates to help keep them in check without getting their hands dirty.
The hardest prisoners in a group called The Program order fellow convicts to brawl in order to get ‘graces’ such as baseball and food privileges.
Those who refuse to fight face being beaten to death, such as 18-year-old Christopher Robinson, who died in 2008.
Rikers Island, which in 2011 incarcerated former International Monetary Fund chief Dominique Strauss-Kahn, also has a cruel reputation for mentally ill prisoners.
More than a dozen have hanged themselves over the last decade.
4. Bang Kwang Prison, Bangkok, Thailand
Bang Kwang Prison in the Thai capital is nicknamed the Bangkok Hilton – but it is far from a luxurious place to spend the night.
Dozens of death row inmates – all wearing welded-on shackles - are crammed into tiny cells and given just two hours notice before they are executed.
Briton John Davies, 74, who spent two years there during the 1990s while serving 17 years for a drugs crime he says he didn’t commit, says he lived in terror while there.
'I knew 22 people to be executed and on one occasion five men from my prison block were killed in one day,' he said.
'One minute the person would be in the cell next to you and the next they would be dragged off to the firing squad. It was terrifying thinking that might happen to me.'
Today, prisoners at Bang Kwang are executed by lethal injections but the conditions – including routine torture - remain largely the same.
3. La Sante Prison, Paris, France
La Sante – ironically meaning health in French – has one of the worst suicide rates of any prison in the world due to its hostile, sanity-destroying atmosphere.
The jail, in the 14th arrondissement of Paris, has been home to some of France’s most infamous and often violent criminals, including Carlos the Jackal.
In the past, rapes are said to have been a daily event, amid claims of a system where stronger inmates enslave weaker ones.
Many are pushed into clinical depression and suicide. In 2003, 122 inmates took their own lives.
The shocking death rate prompted officials to improve conditions, such as increasing the number of showers prisoners were allowed to take from two a week to one a day.
2. Diyarbakır Prison, Turkey
Diyarbakir Pirson in Turkey – aptly nicknamed Hell by inmates - is one of the cruellest jails in the world.
In the past, prisoners have been known to set themselves on fire rather than endure routine torture by sadistic guards.
During an especially bloody episode in 1996, ten inmates were beaten to death and 46 were wounded following a riot.
However, the worst period in the jail was between 1981 and 1984 during military rule in Turkey, when 24 prisoners lost their lives after being tortured.
Conditions are understood to have improved a little since then.
But in 2010, the prison was again criticised in a report by the European Court of Human Rights.
1. Carandiru Prison, Sao Paulo Brazil
Brazil’s Carandiru prison was the deadliest jail in the world - with a total of 1,300 prisoners killed by fellow convicts, guards and a rampant Aids epidemic.
The bloodiest single incident was the 1992 massacre, when military police stormed the dungeon and shot dead 113 inmates – mostly at close range – following a riot.
Former prisoner Jacy de Oliveira told the BBC: 'We never thought they would come in and kill people randomly, as not everyone had joined the rebellion.
'The policemen began shooting everyone. I was on the fifth floor. If you looked a policeman in the eyes, you were dead.'
The jail was closed in 2002, shortly after it was one of 27 prisons in Sao Paulo to hold simultaneous uprisings in which thousands of visitors were held hostage.